U.S. patent number 8,227,765 [Application Number 12/823,318] was granted by the patent office on 2012-07-24 for electrospray pneumatic nebuliser ionisation source.
This patent grant is currently assigned to Microsaic Systems PLC. Invention is credited to Richard Syms.
United States Patent |
8,227,765 |
Syms |
July 24, 2012 |
Electrospray pneumatic nebuliser ionisation source
Abstract
This invention provides a method of combining an array-type
nanospray ionization source comprising a set of externally wetted
proud features and a contact electrode with a pneumatic nebuliser
acting to enhance the flux of sprayed ions. Methods of fabricating
a substrate combining a set of proud features with analyte delivery
and gas flow channels in silicon-based materials are described.
Inventors: |
Syms; Richard (London,
GB) |
Assignee: |
Microsaic Systems PLC (Working,
Surrey, GB)
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Family
ID: |
41008681 |
Appl.
No.: |
12/823,318 |
Filed: |
June 25, 2010 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20110000986 A1 |
Jan 6, 2011 |
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Foreign Application Priority Data
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Jul 3, 2009 [GB] |
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0911554.4 |
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Current U.S.
Class: |
250/425; 250/281;
250/306; 250/286; 250/282; 250/284; 250/522.1; 250/288;
250/423R |
Current CPC
Class: |
H01J
49/0018 (20130101); H01J 49/167 (20130101) |
Current International
Class: |
F23D
11/00 (20060101); H01J 49/04 (20060101); F23D
11/32 (20060101) |
Field of
Search: |
;250/281,282,284,286,288,306,423R,425,522.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
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Spect. 32, 677-688 (1997). cited by other .
Wachs, T., Henion J. "Electrospray device for coupling microscale
separations and other miniaturized devices with electrospray mass
spectrometry" Anal. Chem. 73, 632-638 (2001). cited by other .
Wilm, M., Mann, M. "Analytical properties of the nanoelectrospray
ion source" Anal. Chem. 68,1-8 (1996). cited by other .
Ramsey R., Ramsey J."Generating electrospray from microchip devices
using electro-osmotic pumping" Anal. Chem. 69, 1174-1178 (1997).
cited by other .
Ramsey J.M., Ramsey, R.S. "Material transport method and apparatus"
US 6,231,737 Nov. 16, 1999. cited by other .
Schultz G.A., Corso T.N., Prosser S.J., Zhang S. "A fully
integrated monolithic microchip electrospray device for mass
spectrometry" Anal. Chem. 72, 4058-4063 (2000). cited by other
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Tang K., Lin Y., Matson D.W., Kim T., Smith R.D. "Generation of
multiple electrosprays using micro-fabricated emitter arrays for
improved mass spectrometric sensitivity" Anal. Chem. 73, 1658-1663
(2001). cited by other .
Zhang B., Liu H., Karger B.L., Foret F. "Micro-fabricated devices
for capillary electrophoresis-electrospray mass spectrometry" Anal.
Chem. 71, 3258-3264 (1999). cited by other .
Syms R.R.A., Zou H., Bardwell M., Schwab M.-A. "Microengineered
alignment bench for a nanospray ionisation source" J. Micromech.
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evaluation of existing hardware and emerging technologies" AIAA
97-3058 (1997). cited by other .
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activities" AIAA-2002-5714 (2002). cited by other .
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electrospray ionization source for integration with microfluidics"
Anal. Chem. 74, 5897-5901 (2002). cited by other .
Lozano P., Marinez-Sanchez M. Ionic liquid ion sources:
characterization of externally wetted emitters' J. Colloid Sci.
282, 415-421 (2005). cited by other .
Garza T.G., Lozano P., Velasquez-Garcia L.F., Sanchez M.M. "The
characterization of silicon wettability and properties of
externally wetted micro-fabricated electrospray thruster arrays"
Proc. 29th IEPC, Princeton, Oct. 31-Nov. 4, Paper 195 (2005). cited
by other .
Velasquez-Garcia L.F., Akinwande A.I., Martinez-Sanchez M. "A
micro-fabricated linear array of electrospray emitters for thruster
applications" J. Microelectromech. Syst. 15, 1260-1271-1280
(2006a). cited by other .
Velasquez-Garcia L.F., Akinwande A.I., Martinez-Sanchez M. "A
planar array of micro-fabricated electrospray emitters for thruster
applications" Microelectromech. Syst. 15, 1272-1280 (2006b). cited
by other .
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Martinez-Sanchez M. "Fabrication of a fully integrated electrospray
array with applications to space propulsion" Proc. MEMS 2008,
Tucson, AZ, Jan. 13-17, pp. 976-979 (2008). cited by other .
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isotropic etch by inductively coupled plasma etcher for silicon
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(2006). cited by other .
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P. "SU-8: a low-cost negative resist for MEMS" J. Micromech.
Microeng. 7, 121-124 (1997). cited by other .
Hynes A.M., Ashraf H., Bhardwaj J.K., Hopkins J., Johnston I.,
Shepherd J.N. "Recent advances in silicon etching for MEMS using
the ASETM process" Sensors and Actuators 74, 13-17 (1999). cited by
other .
Robbins H., Schwartz B. "Chemical etching of silicon. I. The system
HF, HNO3 and H2O" J. Electrochem. Soc. 106, 505-508 (1959). cited
by other .
Lee D.B. "Anisotropic etching of silicon" J. Appl. Phys. 40,
4569-4574 (1969). cited by other .
Van Den Meerakker J.E.A.M., Elfrink R.J.G., Roozeboom F. "Anodic
silicon etching: the formation of uniform arrays of macropores or
nanowires" Phys. Stat. Sol a 197, 57-60 (2003). cited by other
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Jansen H., De Boer M., Legtenberg R., Elwenspoek M. "The black
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115-120 (1995). cited by other .
British Search Report from priority application No. GB0911554.4.
cited by other.
|
Primary Examiner: Vanore; David A
Attorney, Agent or Firm: Bishop & Diehl, Ltd.
Claims
The invention claimed is:
1. An electrospray pneumatic nebuliser ionisation source comprising
a substrate having a first side and a second side, the first side
comprising at least one proud wettable feature defined thereon, the
proud feature being in electrical contact with an electrode, the
source additionally comprising a channel extending from the second
side through to the first side for transport of a nebuliser gas to
the first side.
2. The source of claim 1 comprising a table defined on the first
side, the at least one proud wettable feature being provided on the
table.
3. The source of claim 1 wherein the first side is wettable so as
to allow for the flow of a liquid across the first side to the at
least one proud wettable feature.
4. The source of claim 1 wherein the at least one proud wettable
feature has a tip diameter less than 50 microns.
5. The source of claim 1 wherein the substrate is wettable by a
liquid analyte so operably an analyte can be provided to wet the at
least one proud feature.
6. The source of claim 1 wherein the at least one proud wettable
feature is configured to concentrate an electric field.
7. The source of claim 1 configured for operable use with a second
electrode, the second electrode being remote from the first
electrode and wherein the first and second electrodes operably
cooperate to define an electric field therebetween.
8. The source of claim 1 wherein the electric field operably
effects generation of a Taylor cone extending from the at least one
proud feature.
9. The source of claim 8 comprising a plurality of proud features,
each feature operably having a Taylor cone extending therefrom to
emit a flux of ions by electrospray.
10. The source of claim 1 wherein the channel extends at least
substantially about the perimeter of the at least one proud
feature.
11. The source as claimed in claim 10 wherein the channel extends
fully about the perimeter of the at least one proud feature.
12. The source of claim 1 comprising a plurality of proud features
arranged in an array, and wherein the channel extends at least
partially about the perimeter of the array.
13. The source as claimed in claim 12 wherein the channel extends
fully about the perimeter of the array.
14. The source of claim 1 wherein the channel is provided relative
to the at least one proud feature such that operably the channel
provides a gas flow that is coaxial to the electrosprayed ion flux
so as to provide a pneumatic nebuliser.
15. The source of claim 1 wherein the channel is configured such
that operably the transported gas flow is at least partly annular
and at least partly surrounding the electrosprayed ion flux.
16. The source of claim 1 further comprising a second channel
extending from the second side of the substrate to the first side
of the substrate, the second channel operably providing for
transport of a liquid analyte.
17. The source as claimed in claim 16 comprising a plurality of
proud features and wherein the second channel is centred amongst
the proud features, to minimise the time taken by the liquid to
reach each proud feature.
18. The source of claim 1 being configured for direct supply of the
liquid analyte to the first side of the substrate.
19. The source of claim 1 in which the substrate material is
silicon.
20. The source as claimed in claim 19 wherein wettable portions of
the first side are defined by provision of a surface layer of
silicon dioxide on those portions.
21. The source of claim 1, in which the proud features are formed
by an etching process.
22. The source of claim 1 in which the first channel is formed by
etching.
23. The source of claim 1 comprising a surface barrier extending
between a wettable portion and a non-wettable portion of the first
side, the surface barrier operably containing a liquid analyte
within the wettable portion.
24. The source of claim 23 comprising a table defined on the first
side, the at least one proud wettable feature being provided on the
table.
25. The source of claim 24 wherein the surface barrier extends at
least partially about the table so as to contain liquid to that
table portion of the first side.
26. The source of claim 23 wherein the surface barrier is a
polymer.
27. The source of claim 1 wherein the substrate is fabricated in
plastic or ceramic or metal with portions of the substrate coated
with a wettable material to define wettable portions of the
source.
28. The source as claimed in claim 27 wherein the substrate is a
moulded substrate.
29. An electrospray ionisation array comprising a plurality of
sources as claimed in claim 1 arranged relative to one another to
define the array.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of United Kingdom Patent
Application Serial No. GB0911554.4 filed on Jul. 3, 2009.
TECHNICAL FIELD OF THE INVENTION
This invention relates to an electrospray pneumatic nebuliser
ionisation source and in particular to the provision of a
micro-fabricated nanospray ion source that may enhance the spray
from a number of externally wetted proud features using a pneumatic
nebuliser. In a preferred application such a source may be used in
the context of mass spectrometry.
BACKGROUND OF THE INVENTION
Electrospray is a popular method of soft ionisation in mass
spectrometry, since it allows the analysis of fluid samples
pre-separated by liquid chromatography or capillary
electrophoresis, the ionization of complex molecules without
fragmentation, and a reduction in the mass-to-charge ratio of heavy
molecules by multiple charging [Gaskell 1997].
The principle of electrospray is simple. A voltage is applied
between an electrode typically consisting of a diaphragm containing
an orifice and a capillary needle containing a liquid analyte.
Liquid is extracted from the capillary tip and its free surface is
drawn into a Taylor cone, from which large charged droplets are
emitted. The droplets are accelerated to supersonic speed,
evaporating as they travel. Coulomb repulsion of the charges in the
shrinking droplet results in fragmentation to ions when the
Rayleigh stability limit is reached. The resulting ions can be
multiply charged.
Additional methods are used to promote a well-dispersed spray of
small droplets and hence a concentrated flow of analyte ions. Often
these are based on pneumatic nebulisation by a coaxial gas stream,
and a variety of pneumatic nebulisers have been demonstrated [U.S.
Pat. No. 4,746,068; Wachs 2001; U.S. Pat. No. 6,478,238].
Aerodynamic effects such as the Coanda and Venturi effects are also
used to improve the efficiency of ion transmission towards the
inlet of a subsequent analyser such as a mass spectrometer [WO
00/64591; U.S. Pat. No. 6,992,299].
In a conventional electrospray system, with capillaries of around
100 microns internal diameter, flow rates are of the order of 1
micro-litre per minute, and extraction voltages lie in the range
2.5 kV-4 kV. Flow rates and voltages are considerably reduced in
so-called "nanospray systems" [Wilm 1996], based on capillaries
having internal diameters ranging down to around 5 micron [U.S.
Pat. No. 5,115,131; U.S. Pat. No. 5,788,166]. Decreasing the
capillary diameter and lowering the flow rate also tends to create
ions with higher mass-to-charge ratio.
Considerable progress has been made in integrating nanospray
ionisation sources with chip-based separation devices. For example,
an ion spray can be drawn from the edge of a glass chip containing
a capillary electrophoretic separator [Ramsey 1997; U.S. Pat. No.
6,231,737]. Since then, similar sources have been demonstrated in
many materials, especially plastics. Geometries in which the
analyte flows through a capillary etched perpendicular to the
surface of a silicon chip have also been demonstrated [Schultz
2000; U.S. Pat. No. 6,723,985]. Such devices may be formed into
two-dimensional arrays, and it has been shown they can provide an
increased ion-flux based on the ion streams derived from many
separate nanospray sources [Tang 2001, U.S. Pat. No. 6,831,274].
Nebulisers have also been provided for chip-based nanospray
sources, for use with integrated capillaries [Zhang 1999] and with
inserted capillaries [Syms 2007]. However, pneumatic nebulisers
have not so far been used with array-type sources, reducing the
potential advantage of the use of an array.
Capillary electrospray sources have also been considered for use in
so-called colloidal thrusters, a method of micro-propulsion or
attitude adjustment of spacecraft based on the ejection of ions
from capillaries [Mueller 1997; Muller 2002]. In some cases the
devices have been micro-fabricated in silicon [U.S. Pat. No.
6,516,604].
The use of a capillary with a small internal diameter as a source
for nanospray suffers from a number of disadvantages. These include
the difficulty of fabricating suitably fine features, especially in
an integrated device, the likelihood of clogging of such features
by particulate matter or deposits, and problems with matching flow
rates to pre-separation sources of liquid analyte such as liquid
chromatography systems.
One solution to the problem of forming and using a capillary source
with a very small internal diameter is to include a porous bead
inside a larger capillary at its tip [U.S. Pat. No. 5,975,426].
Similarly, one solution to the problem of flow rate matching is to
include inside the capillary a wick element containing an aggregate
of parallel, wettable fibers [U.S. Pat. No. 6,297,499] or nanowires
[U.S. Pat. No. 7,141,807].
While these solutions purport to address the aforementioned
problems there is still a need for improved ionisation sources.
SUMMARY OF THE INVENTION
These and other problems are addressed in accordance by the present
teaching by providing an electrospray pneumatic nebuliser
ionisation source. Such a source combines a pneumatic nebuliser
with emitters, such as nanospray emitters, to provide a high-flux
ion source for liquid analytes, something that has particular
application mass spectrometry.
The pneumatic nebuliser is desirably provided as a coaxial
nebuliser that is combined with a two dimensional array of
externally wetted nanospray emitters. Such a plurality of emitters
are desirably configured and arranged relative to one another such
that each emitter acts as an independent emitter--the array thereby
being formed from a plurality of individual emitters. In a
preferred arrangement such a device is provided using planar
fabrication methods.
The use of a pneumatic nebuliser improves dispersion of the spray
and hence provides an enhanced ion flux.
In a first arrangement, an array of externally wetted nanospray
emitters with a pneumatic nebuliser is provided. Such an array may
be constructed by reactive plasma etching of the front side of a
silicon substrate to form a set of proud features, features that
are upstanding from the surface of the silicon substrate. The
silicon surface may then be surface treated, for example by coating
it with a silicon dioxide layer, to allow wetting by a liquid
analyte so as to flow over the proud features. In a first
arrangement, the liquid can be delivered directly to the front of
the substrate. In another arrangement, the liquid could be
delivered through the substrate onto the proud features using for
example an etched hole or channel provided within the
substrate.
Conducting surface electrodes are typically provided to allow
electrical contact to the liquid. To confine the provided liquid to
defined areas of the substrate, typically that region about the
proud features, a hydrophobic barrier may be provided.
Electrospray may be carried out using a potential difference
applied between the surface electrode and an external electrode.
The form of the generated spray may be enhanced through use of a
nebuliser gas. Typically this is delivered by providing a channel
etched around the emitter array so as to allow a nebuliser gas to
pass through the substrate and provide a concentric gas flow around
the flux of electrosprayed ions.
Accordingly there is provided a device as claimed in claim 1.
Advantageous embodiments are provided in the claims thereto.
These and other features and advantages relating to an exemplary
arrangement provided in accordance with the present teaching will
be better understood with reference to the following figures:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a plan view of a micro-fabricated externally wetted
electrospray ionisation source with an integrated pneumatic
nebuliser.
FIG. 2 shows a section view along the line A-A' of the operation of
a micro-fabricated externally wetted electrospray ionisation source
with an integrated pneumatic nebuliser.
FIG. 3 shows a section view along the line A-A' of a mounting
supplying both nebuliser gas and analyte liquid.
FIG. 4 shows a section view along the line A-A' of a mounting
supplying a nebuliser gas, with the analyte liquid being applied
externally.
FIG. 5 shows a section view along the line A-A' of a fabrication
process for forming an array of externally wetted proud features
for electrospray emission, wherein FIG. 5A illustrates step 1 of
the process, FIG. 5B illustrates step 2 of the process, and FIG. 5C
illustrates step 3 of the process.
FIG. 6 shows a section view along the line A-A' of a fabrication
process for forming a conducting surface electrode, wherein FIG. 6A
illustrates step 4 of the process, FIG. 6B illustrates step 5 of
the process, and FIG. 6C illustrates step 6 of the process.
FIG. 7 shows a section view, along the line A-A' of a fabrication
process for forming a gas flow channel and a liquid flow channel,
wherein FIG. 7A illustrates step 7 of the process, FIG. 7B
illustrates step 8 of the process. FIG. 7C illustrates step 9 of
the process, and FIG. 7D illustrates step 10 of the process.
DETAILED DESCRIPTION
An exemplary arrangement provided to assist in an understanding of
the present teaching will now be described with reference to FIGS.
1 to 4. FIG. 1 shows a plan view of a micro-fabricated externally
wetted electrospray ionisation source with an integrated pneumatic
nebuliser, according to this exemplary arrangement. The device is
formed in a planar substrate 101. It will be appreciated that
fabrication using standard micro-fabrication techniques is
advantageous. So as to allow for the use of standard
micro-fabrication processes, the substrate material is desirably
silicon which may be coated in a hydrophilic layer to allow wetting
by aqueous solutions. Suitable hydrophilic surface coatings include
silicon dioxide, but it will be understood that it is not intended
to limit the present teaching to such treatments.
A central table 102 is formed in and on a first side of the
substrate by etching a slot 103 through the substrate to form a
perimeter that almost completely surrounds the table, allowing the
table to remain attached to the substrate by a number of narrow
sections of material 104. The etched slot 103 provides a channel
from the rear of the substrate to the front of the substrate,
through which gas may subsequently flow, while the narrow sections
104 provide a mechanical support for the central table against the
gas flow.
The central table carries a number of proud features 105, each
proud feature being formed by etching the first side, which may be
considered a front side, of the substrate. Each of the proud
features desirably form an individual emitter. The shape of each
feature is desirably such as to allow for efficient generation of
an electrospray which is desirably effected by externally wetting
each of the provided emitters. In practise, the height of the proud
features will be limited to around 100 microns, and the diameter of
each proud feature at its tip will be chosen so that the resulting
emitter can operate in the nanospray flow regime, typically 1
micron-50 micron.
In operation it is desirable for the proud features to have a
liquid coating so as to have a wettable surface. Such a liquid 106
(here, presumed to be a liquid analyte) may be provided to coat the
hydrophilic surface in the vicinity of the table centre and arising
from the geometrical construction of the device will also flow over
the proud features 105. The liquid 106 may be delivered directly to
the front side of the substrate as described below or from a second
side, in this case the rear side, of the substrate via an
additional hole 107 etched right through the substrate.
Electrical contact to the liquid is provided by a conducting
surface electrode 108 on the front side of the substrate, which is
connected to a contact pad 109 on the substrate 101 by a section of
conducting track 110 desirably passing over one of the short
sections of material 104. Suitable conducting materials include
noble metals such as gold, or other materials as will be
appreciated by those of ordinary skill in the art
If the conducting surface electrode 108 is hydrophobic and provided
in the geometry of a closed ring, it may act as a confining barrier
to prevent unwanted flow of liquid over the remainder of the
substrate. Alternatively, a confining liquid barrier may easily be
provided using a ring of an alternative hydrophobic material 111.
Suitable barrier materials include hydrophobic polymers.
FIG. 2 shows a section view along the line A-A' of the operation of
a micro-fabricated externally wetted electrospray ionisation source
with an integrated pneumatic nebuliser. In this case, delivery of
the liquid 106 is assumed to be from the rear of the substrate 101,
passing through the etched channel 107 described above with
reference to FIG. 1 so as to pass from the second side to the first
side. The substrate 101 is coated with a layer 201 that can be
wetted by the liquid, so that it can flow over the proud features
105 to contact the confining surface electrode 108. However through
the arrangement provided on the top surface of the substrate, the
liquid is confined within the hydrophobic ring 111.
A potential difference derived from a voltage source 202 is applied
between the electrode contact 109 and an external electrode 203.
The external electrode typically contains at least one aperture 204
to allow ions to pass through to a subsequent analytic device.
Examples of subsequent devices include mass spectrometers with
atmospheric pressure ion sampling.
The potential difference is sufficiently large that the liquid film
coating each proud feature 105 may be drawn out into a Taylor cone.
Each proud feature may then emit a stream of ions 205 by
electrospray, and the combined ion stream will thereby present a
concentrated flux of electrosprayed ions. In this way, the
plurality of proud features generates a multi-emitter. A flow of
gas 206 may be passed from the rear of the substrate to the front
via the etched slot 103, thus providing a concentric flow of gas
around the ion streams to promote nebulisation and hence to further
enhance the overall ion flux.
Thus it will be appreciated that the overall construction described
provides an exemplary method of combining a set of externally
wettable electrospray emitters, a contacting electrode, a channel
for a concentric gas flow and a liquid input channel on a planar
substrate.
It will also be appreciated that the arrangement shown is
representative and not restrictive. For example, different numbers
of externally wettable emitters may be used, with different sizes,
spacing and general arrangement to that shown, without departing
from the present teaching. In general the spacing of the externally
wettable emitters will be chosen to provide a close packed array,
so that a large ion density is achieved, and the number will be
chosen to provide an emission rate matching the rate of delivery of
the liquid analyte.
It will also be appreciated that different numbers of substrate
sections may be used to support the central table, with different
sizes, spacing and general arrangement to that shown. In general,
the arrangement of these sections will be chosen to provide a
mechanical restraint against the nebuliser gas flow and to allow
one or more electrical pathways between the conducting surface
electrode and contact pads on the substrate.
It will also be appreciated that the gas flow channel will
typically at least partially surround the central table, so that a
mainly concentric flow of nebuliser gas may be provided. The gas
flow may be delivered as shown in the example arrangement of FIG.
3. Here the electrospray source 301 is attached to a mounting 302
by a planar gasket 303, which provides a gas and liquid tight seal
therebetween. Suitable gasket materials include the elastomer
polydimethyl-siloxane (PDMS) which may conveniently be formed into
a wide range of shapes by casting or moulding, and which may form a
secure bond to a wide range of materials including silicon and
silica after suitable surface treatment. The arrangement may
provide an input channel for a stream of nebuliser gas 304.
It will also be appreciated that the liquid input channel will
desirably be placed at the centre of the array of proud features,
so that the time taken for the liquid to flow over the proud
features is minimised. In this case, peak-broadening effects will
be reduced when the liquid analyte is derived from a pre-separation
device such as a liquid chromatography system. The liquid flow may
also be provided as shown in the example arrangement of FIG. 3.
Here the planar sealing gasket is arranged to provide a socket for
mounting a capillary 305 carrying the flow of liquid 306.
It will also be appreciated that under some applications it may be
desirable to omit the liquid channel, and instead deliver the
analyte liquid directly to the front side of the device. FIG. 4
shows an arrangement where the surface coating liquid 106 is
dispensed as droplets from a liquid analyte 401 contained in a
capillary 402. This arrangement may be particularly convenient when
more than one electrospray source is being used in conjunction with
more than one liquid analyte, the liquids being supplied from
capillaries for example by a robotic sampler.
FIGS. 5-7 show an exemplary construction of the device of FIGS. 1
to 4 using standard micro-fabrication methods, using a silicon
substrate as a starting material.
FIG. 5 shows a method of forming an array of proud features with a
wettable surface. In step 1, a silicon substrate 101 is first
coated in a layer of masking material 501, which is patterned to
define a set of masking features 502, each masking feature being
placed at the desired location of one proud feature. Suitable
masking layers include photoresist patterned using optical
lithography, and silicon dioxide patterned by reactive ion etching
using an additional surface mask of photoresist patterned by
lithography.
In step 2, the exposed silicon surface is etched, using a process
that etches laterally as well as vertically, so that the mask layer
is undercut and a set of proud features 503 with a tapered profile
surrounded by wells 504 is formed. Suitable etching methods for a
silicon substrate include isotropic reactive ion etching, using
plasma containing SF.sub.6 gas [Ji 2006]. The dimensions of the
masking features and the etching time are chosen so that the proud
features are formed with a suitable height and a suitable tip
diameter. Depending on the relative isotropy of the etching
process, the proud features may be in the form of cone shapes with
different apex angles, or have a surface that has more than one
radius of curvature.
In step 3, the surface mask is removed, and the silicon is coated
with a thin layer of a wettable material 505 to form the wettable
proud features 105. Suitable wettable materials include silicon
dioxide, and suitable layer deposition methods include thermal
oxidation, chemical vapour deposition and plasma enhanced chemical
vapour deposition. Methods of enhancing the wettability of silicon
dioxide include immersion in water for a period of time.
FIG. 6 shows a method of forming a contact electrode with a
hydrophobic barrier. In step 4, the front hydrophilic layer 505 of
the substrate 101 is conformally coated with a layer of conducting
metal 601, which is then coated in a layer of masking material 602
that defines a set of electrode features. Suitable conducting
materials include noble metals such as gold, and suitable
deposition methods include radio frequency sputtering. Suitable
masking materials include photoresist patterned by lithography.
In step 5, the conducting layer is etched to transfer the pattern
of the surface mask 602 into a set of corresponding conducting
features such as the contacting electrode 108 in the underlying
conducting layer. The surface mask is removed to leave the surface
of the device exposed.
In step 6, further lithography is carried out to incorporate
three-dimensional hydrophobic features to restrict wetting in the
case when large volumes of liquid are dispensed and confine any
surface wetting liquid to the vicinity of the proud features. An
additional layer of a hydrophobic polymer is deposited and
patterned using lithography into the shape of a dam 111 surrounding
the region of the table containing the proud features. Suitable
photopatternable hydrophobic polymers include the permanent epoxy
photoresist SU-8 [Lorenz 1997].
FIG. 7 shows a method of forming the gas flow channel, and also a
liquid flow channel if desired. In step 7 the surface of the device
is coated with a layer of masking material 701 that defines a set
of channels. Suitable masking materials include a thick layer of
photoresist patterned by lithography.
In step 8, the hydrophilic layer 505 and the substrate 101 are
sequentially etched to transfer the pattern of the surface mask
into a corresponding set of channels, for example a gas flow
channel 103 and also a liquid flow channel 107 if desired. An
additional surrounding channel may also be cut round the majority
of the device, so that the device may be separated from the wafer
by fracturing a short section of silicon, without the need for
dicing. Any rear coating 702 that might have been deposited during
oxidation is also etched sequentially using the same mask pattern
and an appropriate etching method. Suitable etching methods for a
hydrophilic layer based on silicon dioxide include anisotropic
reactive ion etching using plasma containing a mixture of
CHF.sub.3, O.sub.2 and Ar gases. Suitable etching methods for a
silicon substrate include deep reactive ion etching in a
high-density plasma using a process such as the one developed by
Robert Bosch GmbH [U.S. Pat. No. 5,501,893; Hynes 1999]. This
process is based on alternating cycles of silicon etching using
plasma containing SF.sub.6 and O.sub.2 and of sidewall passivation
using plasma containing C.sub.4F.sub.8 gas.
In step 9, the surface mask is removed to leave the surface of the
device exposed. In step 10, the device is cleaned, separated from
the rest of the wafer, and a bond wire 703 is attached to the
contact pad 109 to allow application of an electrical
potential.
The fabrication process described above is intended to be
exemplary, and for use with a silicon starting substrate. It will
be appreciated to those knowledgeable in the art of
microfabrication that the order of the individual process steps may
be permuted to yield a similarly compatible process without
substantially altering the final result, and that other equivalent
process steps may be used to replace the process steps
described.
For example, different etching processes may be substituted to form
the proud features using a surface mask. Examples include isotropic
wet chemical etching of silicon [Robbins 1959], anisotropic wet
chemical etching of silicon [Lee 1969], and anodic etching of
silicon [van den Meerakker 2003]. Other methods of etching may also
be used that form large numbers of proud features without the need
to mask each feature separately, for example the `black silicon
method` which can form a rough or grassy silicon surface by careful
choice of the etching conditions [Jansen 1995]. Different processes
may also be substituted to etch the silicon channels. Examples
include cryogenic deep reactive ion etching.
Different coating processes may be substituted to form a
hydrophilic silicon dioxide layer. Examples include RF sputtering
of SiO.sub.2 and anodic oxidation. Different etching processes may
be substituted to etch the silicon dioxide layer(s). Examples
include wet chemical etching in buffered HF.
It will be apparent to those skilled in the art that other
processes may be used with different starting materials, to yield a
similar final object. For example, the main structural features may
be formed by replica moulding of a plastic or a ceramic, or by
electroplating a metal inside a mould. In each case the master may
conveniently be formed using silicon-based planar processing. A
hydrophilic silicon dioxide coating may then be incorporated, using
RF sputtering. Similarly, a contacting electrode structure may be
formed by evaporation of a metal through a stencil.
It will also be appreciated that whatever process is used for
fabrication, a plurality of similar externally wettable
electrospray sources may be constructed as an array. Such arrays
may used in applications where different sources are required to
spray different analytes in turn, or where redundancy is required
to allow for the possibility of device failure.
It will be understood that what has been described herein are
exemplary embodiments of an ionisation source comprising a
pneumatic nebuliser to enhance ion flux. Such a source has
application in a number of different fields, exemplary applications
having been described with reference to mass spectrometry. It will
however be understood that it is not intended to limit the present
invention in any way except as may be deemed necessary in the light
of the appended claims.
Within the context of the present invention the term
microengineered or microengineering or micro-fabricated or
microfabrication is intended to define the fabrication of three
dimensional structures and devices with dimensions in the order of
microns. It combines the technologies of microelectronics and
micromachining Microelectronics allows the fabrication of
integrated circuits from silicon wafers whereas micromachining is
the production of three-dimensional structures, primarily from
silicon wafers. This may be achieved by removal of material from
the wafer or addition of material on or in the wafer. The
attractions of microengineering may be summarised as batch
fabrication of devices leading to reduced production costs,
miniaturisation resulting in materials savings, miniaturisation
resulting in faster response times and reduced device invasiveness.
Wide varieties of techniques exist for the microengineering of
wafers, and will be well known to the person skilled in the art.
The techniques may be divided into those related to the removal of
material and those pertaining to the deposition or addition of
material to the wafer. Examples of the former include:
Wet chemical etching (anisotropic and isotropic)
Electrochemical or photo assisted electrochemical etching
Dry plasma or reactive ion etching
Ion beam milling
Laser machining
Excimer laser machining
Whereas examples of the latter include:
Evaporation
Thick film deposition
Sputtering
Electroplating
Electroforming
Moulding
Chemical vapour deposition (CVD)
Epitaxy
These techniques can be combined with wafer bonding to produce
complex three-dimensional, examples of which are ionisation source
devices as heretofore described.
Where the words "upper", "lower", "top", bottom, "interior",
"exterior" and the like have been used, it will be understood that
these are used to convey the mutual arrangement of the layers
relative to one another and are not to be interpreted as limiting
the invention to such a configuration where for example a surface
designated a top surface is not above a surface designated a lower
surface.
Furthermore, the words comprises/comprising when used in this
specification are to specify the presence of stated features,
integers, steps or components but does not preclude the presence or
addition of one or more other features, integers, steps, components
or groups thereof.
* * * * *